Arrangement for wide-band coupling of the spiral line of a travelling-field spiral tube to a coaxial line



Jan. 7, 1969 5 ETAL- 3,421,120

ARRANGEMENT FOR WIDE-BAND COUPLING OF THE SPIRAL LINE OF ATRAVELLING-FIELD SPIRAL TUBE TO A COAXIAL LINE Filed July 9, 1964 UnitedStates Patent gesellschaft, Munich, Germany, a corporation of GermanyFiled July 9, 1964, Ser. No. 381,585 US. Cl. 33331 Int. Cl. H03h 7/30 3Claims ABSTRACT OF THE DISCLOSURE An arrangement for the wide-bandcoupling of the main spiral line of a travelling-wave tube with anessentially metallic tubular vacuum-tight envelope to a coaxial line,serving for the feed-in or lead-off of highfrequeney energy, mountedperpendicularly to the tube, with the outer conductor of the coaxialline being extended to and merged in the metallic envelope, while theinner conductor of the coaxial line is conductively connected over thetransformation spiral with the last coil of the main spiralcharacterized by the feature that the pitch and diameter of thetransformation spiral is greater than that of the main spiral, saidtransformation spiral comprising at least one section and having, atleast section-wise, constant pitch, said transformation spiralcomprising a conductor having substantially uniform crosssectionaldimensions throughout its length.

The disclosure The invention relates to an arrangement for wide-bandcoupling of the spiral line of a travelling-wave tube with anessentially metallic tubular vacuum-tight envelope to a coaxial lineserving for the feed-in or lead-off of highfrequency energy, in whichthe coaxial line is mounted perpendicularly to the tube, with the outerconductor thereof extended .to and merged in the metal envelope, whilethe inside conductor of the coaxial line is conductively connected overa spiral conductor portion (transformation spiral) with the last or endcoil of the spiral line.

In travelling-wave tubes it is a known practice to couple the spiralline to a coaxial line serving for the tfeed-in or lead-off ofhigh-frequency energy, in which coaxial line to the inner conductor isdirectly connected conductively with the end of the spiral line. Theouter conductor of the coaxial line is connected, in the knownarrangements of this type, with a cylindrical metal body which surroundsthe spiral end portion. This cylindrical metal body, whose inner surfaceis generally horn-shaped or conical, should be designed and arranged insuch a way that the smallest distance of the spiral from the innersurface of the metal body is less than a quarter of the axial wavelengthat the corresponding point of the spiral.

In the known arrangements described, the required small distance betweenthe metal body and the spiral necessitates very close tolerances andthereby leads to technological difficulties. -In order to reduce thisdifliculty, it is a known practice to artificially enlarge the axialwavelength of the spiral in the region of the transition portions to thecoaxial line, by increasing the pitch of the spiral thereat. In order toachieve an adaptation, as good as possible, of the spiral to the innerconductor of the coaxial line, it is necessary for the transition fromthe portion of increased .pitch at the end of the spiral to the normalpitch of the spiral line to gradually become smaller and smaller. Thisnecessitates the utiliza- 3,421,120 Patented Jan. 7, 1969 tion of aspiral portion with variable pitch. The technological expenditure for anadequately precise manufacture of such a spiralportion with graduallychanging pitch, however, again is very great.

The invention is directed to the problem of creating a wideabandcoupling of the spiral line of a travellingwave spiral tube to a coaxialline without great technological expenditure. For the solution of thisproblem, in an arrangement of the type heretofore mentioned, it isproposed according to the invention, that the transformation spiral witha constant pitch, which in a manner in itself known, has a pitch greaterthan the pitch of the spiral line (main spiral), and a diameter greaterthan that of the main spiral, so that the conductor forming thetransformation spiral extends at a considerably smaller distance fromthe metal envelope than the conductor forming the main spiral.

An arrangement according to the invention has the important advantagethat there is required merely a single transformation member between thecoaxial line and the spiral line, in the form of a transformation spiralof constant pitch, which may be simply and inexpensively produced. Thistransformation spiral can be dimensioned in simple manner as a stripline according to high-frequency technology, which has as base conductorthe metal vacuum envelope of the tube.

-In the drawing, wherein like reference characters indicate like orcorresponding parts:

FIG. 1 is a longitudinal section of the cathode end of a travelling-Wavetube with a coupling device according to the invention; and

FIG. 2 is a transverse cross section thereof.

While FIGS. 1 and 2 illustrate sections of only the cathode end of atravelling-wave tube, the uncoupling device at the collector end of thetube would, in principle, be constructed in the same manner.

In the drawings, those parts of the tube which do not appear necessaryfor an understanding of the invention, such as, for example, the supportclamping of the main spiral and transformation spiral, have beenomitted.

The helix or spiral '1, herein designated as the main spiral, which isto be coupled to the coaxial line 2, is coaxially surrounded by a metaltubular vacuum-tight envelope 3. The outer conductor 4 of the coaxialline sealed vacuum-tight, for example, by a ceramic ring 5, is extendedto and merged directly in the metal vacuum-tight envelope 3 of the tube.The inner conductor 6 of the coaxial line is conductively connecteddirectly with the adjacent end of a short transformation spiral 7, whoseother end in turn is conductively connected with the end coil of themain spiral 1. The transformation spiral 7 has a greater pitch and agreater diameter than the main spiral 1, with the pitch and the diameterof the transformation spiral 7 being constant. The end of thetransformation spiral 7, which, like the main spiral 1 can be supportedunder elastic deformation between three insulating bars bearing directlyon the envelope 3, is integrally connected with the main spiral. Thetransformation spiral 7 and the main spiral .1 advantageously comprise aconductor in the form of a band, dimensions of which are the same forboth spirals.

The wave impedance of the transformation spiral 7 can, with a givenwidth b of the conductor band forming the spirals, in a simple manner beadjusted by suitable selection of the distance h from the metal envelope2 whereby the usual known transformation condition is fulfilled. Thetransformation spiral can here be treated, in high frequency technology,as a strip line in which the envelope of the tube is the base conductorand the conductor band of the transformation spiral is the stripconductor. Consideration in this manner is practically always possibleif the distance it of the conductor band from the base conductor isconsiderably smaller than the pitch Pt of the transformation spiral 7. Astrip line, as is well known, has an extremely wide-band, so that theentire coupling device has wide-band characteristics.

The invention is not limited to the embodiment illustrated. Inparticular, it is possible for the transformation spiral to besubdivided into several sections which have diameters differing from oneanother. It has also been proved that an especially good, wide-bandadaptation of the coupling device is attainable if the transformationspiral 7 consists of two sections, of which the section directlyconnected with the inside conductor of the coaxial line has a greaterdiameter than the section which is adjacent to the main spiral. Thereare thus obtained two successively connected strip lines of differentwave impedance, the wave impedance again being selected in dependenceupon the known transformation conditions involved.

Changes may be made within the scope and spirit of the appended claimswhich define what is believed to be new and desired to have protected byLetters Patent.

We claim:

1. An arrangement for the wide-band coupling of the main spiral line ofa travelling-wave tube with an essentially metallic tubular vacuum-tightenvelope to a coaxial line, serving for the feed-in or lead-off ofhigh-frequency energy, mounted perpendicularly to the tube, with theouter conductor of the coaxial line being extended to and merged in themetallic envelope, while the inner conductor of the coaxial line isconductively connected over a transformation spiral with the last coilof the spiral windings characterized by the feature that the pitch andthe diameter of the transformation spiral are greater than those of themain spiral, said transformation spiral comprising at least one sectionand having section-wise, constant pitch and constant diameter, saidtransformation spiral and said main spiral comprising a band-shapedconductor having substantially uniform cross-sectional dimensionsthroughout its length.

2. An arrangement according to claim 1, wherein the distance from themetal envelope of the conductor forming the transformation spiral issmaller than the pitch of the transformation spiral.

3. An arrangement according to claim 2, wherein the transformationspiral consists of two sections which have different diameters from oneanother, the section adjacent to the coaxial line having a greaterdiameter than the section of the transformation spiral which is adjacentto the main spiral.

References Cited UNITED STATES PATENTS 2,850,704 9/ 1958 Munushian3153.5 2,922,068 1/1960 Kennedy 315-36 FOREIGN PATENTS 841,311 6/1952Germany.

ELI LIEBERMAN, Primary Examiner.

US. Cl. X.R. 3l53.6

